A malfunction management unit (MMU) is a device used in traffic control assemblies to detect and respond to conflicting or otherwise improper signals. Such improper signals may arise, for example, due to field signal conflicts, a malfunctioning controller, faulty load switches, component mis-wiring, improper supply voltages, and the like. Conventional MMUs are unsatisfactory, however, in that they may not be capable of recognizing some types of improper signals. One such type of improper signal relates to what is often referred to as the “yellow trap” sequence.
More particularly, protected/permissive left turn (PPLT) traffic controls are often used to increase the left turn capacity and reduce delay at intersections by providing an exclusive turn phase for left turns (protected) as well as a phase during which left turns can be made as gaps in opposing traffic permit (permissive). The protected left turn can either precede (lead) or follow (lag) the opposing through signal phase.
A major concern with such controls is when the change from permissive left turns in both directions to a lagging protected left turn in one direction. The left-turning driver whose permissive interval is ending may try to proceed through the intersection on the yellow indication, not realizing that the opposing through traffic still has a green through indication. This may occur when the yellow display for the adjacent through movement appears and the left-turning driver ordinarily expects the opposing through display to be yellow as well. The driver may now mistakenly believe that the left turn can be completed on the yellow indication or immediately thereafter when the opposing through display will presumably red. This sequence of events is referred to as the “yellow trap,” and can lead the left turning driver into the intersection when it is possibly unsafe to do so even though the signal displays are correct.
To avoid the yellow trap, many agencies do not use leading/lagging protected/permissive left turns. Given the complexity of modern traffic control equipment, it is often difficult for field operators to correctly program traffic controller units such that actuated signal operation sequences in a safe manner avoiding the yellow trap. While signal displays have been devised to preclude the yellow trap, such as the Flashing Yellow Arrow and a scheme referred to as “Dallas Phasing,” the possibility of a yellow trap occurring is still likely when unskilled technicians mis-program the traffic controller.
Accordingly, there is a need for improved systems and methods for detecting yellow trap sequences as they occur at intersections. These and other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background section.